Surface-enhanced Raman Spectroscopy (SERS) is a sensing technique that may be capable of providing relatively fast, specific and label-free detection of spectroscopic fingerprints of molecules adsorbed on metal surfaces. It has been shown that a substantial Raman enhancement may arise from localized spots, or “hot spots,” in metallic nanostructures owing to concentrated electromagnetic near-fields associated with strong and localized surface plasmon resonances of some metallic nano-constructs. As such, SERS has been performed on electrochemically roughened metal surfaces, colloids, island films, nanowires, periodic arrays, and self-assembled nanoparticles, and has been used for trace detection of chemical and biological targets. Although these nano features are typically spread over an entire surface, the number of hot spots attainable may be limited and inconsistent. As such, novel substrates and methods for performing SERS may be desirable.
The structure of the electrical double layer (EDL) has been extensively studied over the past century. Based on the classical works by Gouy, Chapman, Stern, Frumkin and Grahame (See Grahame, D. C., Chem. Rev., 41 441 (1947)), a converging picture of the structure of the EDL has emerged. The EDL may be made of a surface-localized part (also referred to as the compact layer) and a thermally mobile and spatially distributed part (also referred to as the diffuse layer). The surface-localized part of the EDL may include localized charges, including electrons, solvent molecules, and/or specifically adsorbed ions, and the thermally mobile and spatially distributed part may include various solvated electroactive and inactive ions. These charged species may cause the EDL structure to act as a capacitor. Aside from some recent efforts made to incorporate nanostructures to a surface to achieve super charge capacitors, there has not been significant progress toward detailed understanding of the EDL structure, or toward utilizing this phenomenon for practical applications.
Recently, the inventor of the present application has investigated the structure and effect of the EDL of a nanometer electrode by a finite-element method. See, Yang, X. and Zhang, G, Nanotechnology 18 335201-335209 (2007). This study established that the EDL structure may cause altered current response for nanometer electrodes due to the expansion of the diffuse layer into the diffusion layer, and that the effect of the EDL on the electron transfer and current response of single nanometer electrodes may be significantly influenced by changes in relative-permittivity (or dielectric constant) and compact layer thickness.
In view of the foregoing, novel substrates and methods for practical applications using EDL capacitance may be desirable.